Nuclear fuel is reprocessed in order to separate the uranium, the plutonium, and the fission products present in fuel elements that have spent time in a reactor.
A method known as "vitrification" has been developed to include a certain number of radioactive elements (and in particular fission products) within a vitreous matrix.
Glass frit and calcinates of fission products are simultaneously melted in the crucible of an induction oven in order to obtain a glass. This glass is then cast into stainless steel containers.
This method is imposed since it enables highly active products to be stored over long periods of time with very high security.
This entire method of operation has been widely used, described, and demonstrated, in particular at the Marcoule Nuclear Center (vitrification workshops).
In general, the installation comprises two levels which are separated by a slab. The upper level contains the crucible in an oven, and the lower level contains the container which is disposed vertically below a casting hole from the crucible. An opening through the slab allows casting to take place.
The following problems arise:
the molten glass must be strictly confined so that there is no dissemination of radioactivity, particularly while casting; PA1 all of the gases, dust, and particles that may be given off must be collected; and PA1 the container must be allowed to expand under the effect of increasing temperature (increasing between the empty container and the container full of very hot glass).
The first problem further includes avoiding any contamination on the outside of the container.